Fuel cells have been used as a power source in many applications. For example, fuel cells have been proposed for use in electrical vehicular power plants to replace internal combustion engines. In proton exchange membrane (PEM) type fuel cells, hydrogen is supplied to the anode of the fuel cell and oxygen is supplied as the oxidant to the cathode. PEM fuel cells include a membrane electrode assembly (MEA) comprising a thin, proton transmissive, non-electrically conductive, solid polymer electrolyte membrane having the anode catalyst on one face and the cathode catalyst on the opposite face. The MEA is sandwiched between a pair of non-porous, electrically conductive elements or plates which (1) serve as current collectors for the anode and cathode, and (2) contain appropriate channels and/or openings formed therein for distributing the fuel cell's gaseous reactants over the surfaces of the respective anode and cathode catalysts.
The term “fuel cell” is typically used to refer to either a single cell or a plurality of cells (stack) depending on the context. A plurality of individual cells are typically bundled together to form a fuel cell stack and are commonly arranged in electrical series. Each cell within the stack includes the membrane electrode assembly (MEA) described earlier, and each such MEA provides its increment of voltage. A group of adjacent cells within the stack is referred to as a cluster.
In PEM fuel cells, hydrogen (H2) is the anode reactant (i.e., fuel) and oxygen is the cathode reactant (i.e., oxidant). The oxygen can be either a pure form (O2) or air (a mixture of O2 and N2). The solid polymer electrolytes are typically made from ion exchange resins such as perfluoronated sulfonic acid. The anode/cathode typically comprises finely divided catalytic particles, which are often supported on carbon particles, and mixed with a proton conductive resin. The catalytic particles are typically costly precious metal particles. As such these MEAs are relatively expensive to manufacture and require certain conditions, including proper water management and humidification and control of catalyst fouling constituents such as carbon monoxide (CO), for effective operation.
The electrically conductive plates sandwiching the MEAs may contain an array of grooves in the faces thereof that define a reactant flow field for distributing the fuel cell's gaseous reactants (i.e., hydrogen and oxygen in the form of air) over the surfaces of the respective cathode and anode. These reactant flow fields generally include a plurality of lands that define a plurality of flow channels therebetween through which the gaseous reactants flow from a supply header at one end of the flow channels to an exhaust header at the opposite end of the flow channels.
Interposed between the reactant flow fields and the MEA is a diffusion media serving several functions. One of these functions is the diffusion of reactant gases therethrough for reacting with the respective catalyst layer. Another is to diffuse reaction products, such as water, across the fuel cell. In order to properly perform these functions, the diffusion media must be sufficiently porous while maintaining sufficient strength. The strength is required to prevent the diffusion media from tearing when assembled within the fuel cell stack.
The flow fields are carefully sized so that at a certain flow rate of a reactant, a specified pressure drop between the flow field inlet and the flow field outlet is obtained. At higher flow rates, a higher pressure drop is obtained while at lower flow rates, a lower pressure drop is obtained. However, the pressure drop experienced between the flow field inlet and the flow field outlet may vary from the designed pressure drop. Such variations can be caused by variations in the manufacturing of the fuel cells and stack and/or in the tolerances of the components used in the fuel cells and stacks. Such variations from the designed pressure drop can be detrimental to the operation and/or performance of the fuel cells and stack.
Additionally, fuel cells and stacks can become unstable during low power draws. That is, during a low power requirement of the fuel cells and stack, the flow of reactants through the fuel cells and stack is reduced and the velocity of the reactants through the flow fields decreases which can cause the fuel cell stack to become unstable. One cause of instability is the reduced velocity of the reactant not providing enough shear force or dynamic pressure to transport reaction products (H2O) out of the fuel cells. The inadequate shear force or dynamic pressure may not allow the gaseous reactants clear access to the reacting surfaces (catalyst layers) and may allow water and/or other reactants from the flow fields to build up within the flow channels. One method of improving the low power operation of a fuel cell stack is to design the flow channels to have a higher pressure drop so that during the low power draws a higher flow velocity results. However, this is impractical to do because the pressure drop generally increases linearly with flow rate. Thus, if the pressure drop is increased 10% at a low power point then the pressure drop is also increased 10% at a higher power point. Since pressure drop represents wasted energy, it is not desirable to increase the pressure drop at the higher power outputs of the fuel cell stack. Therefore, what is needed is an improved fuel cell and/or fuel cell stack having an improved flow field design.